1. Field of the Invention
The invention generally relates to the field of signal processing. More specifically the invention is related to estimating powers of user subchannels and interference for the purpose of interference cancellation and error decoding.
2. Discussion of the Related Art
With the advent of new CDMA standards for Data transmission, there has been an ever-growing demand for higher data rates. However, interference degrades signal detection, tracking, and demodulation capabilities of a CDMA receiver by impeding the recovery of a signal of interest. Interference can occur when one or more unwanted signals are received simultaneously with a signal of interest. The interfering signals increase the total energy of the received signal, but decrease the Signal to Noise Ratio (SNR) of the signal of interest. Examples of such interference include multiple-access interference (MAI) and inter-symbol interference (ISI).
ISI can occur in a dispersive (e.g., multipath) environment when a transmitted signal propagates over multiple signal paths to produce an interfering signal path and a selected signal path that differentially arrive at a receiver, thereby hindering processing of the selected signal path. MAI may include interference caused by signal paths emanating from other transmitters, thereby hindering the processing of a signal from a desired transmitter. Although CDMA employs orthogonal multiple-access spreading (e.g., covering) codes to broadcast different messages to different users, multipath delay can disrupt the orthogonality between different coded subchannels. Thus, a receiver may employ interference cancellation to extract a message intended for it from a linear combination of coded signals.
Prior-art interference-cancellation techniques employ symbol estimates for data modulated on coded subchannels for synthesizing an estimated interference signal. For example, symbol estimation is performed on a per-finger basis prior to S-matrix (i.e., interference matrix) generation, which is also known as “SMG.” The estimated interference signal is cancelled from the received signal to produce an interference-cancelled signal.
In prior-art error decoding, the assumption usually made is that the noise and interference power is the same across all subchannels, and the quality of the symbol estimates therefore assumes that the noise observed on the pilot is the same as that for a subchannel of interest.